It seems as though the Greek government is trying its best to prosecute people profiting from promoting unlicensed gambling.
All the government wants to do is prosecute.

Instead of creating conditions (favorable, clear, and reliable ones) that would make casinos come to get licensed voluntarily, rather than running away.
The current government introduced a licensing scheme for casinos and already more than a dozen are operating legally with licenses issued within the country.
A license (call it what you will, but in reality, it is a "payoff" to a powerful entity capable of meting out punishment) represents the fine line between a casino operating illegally and legally.
However, looking to avoid expensive licenses and regulatory procedures many casinos will skip the licensing procedures entirely and just go straight to guerilla promotion with influencers.
This is because they dislike the official "extortion". Regulators shouldn't simply demand payment for the right to operate; they need to offer something in return to the gambling industry to make obtaining a license worthwhile.
Furthermore, the online nature of casinos creates opportunities to evade regulators - opportunities that casinos, naturally, don't pass up.
These casinos change their domains every day or so to avoid blocks playing cat and mouse games with the government.
Because it works out "cheaper" than buying a license. As long as it remains economically advantageous (and opportunities remain), casinos will continue to evade regulators.
Perhaps the owners of these casinos were registering the legal entities abroad and maybe even they themselves were abroad.
For no one wants to be held accountable.

As for Greece, the ministry stated around 900 thousand people last year alone played in illegal casinos. For a country of 10 million people, that surely is significant.
Did you mean a country with high tourist traffic?